okay so after another day of research, on the series of cards in particular rather than the software they run; turns out the R9 series across the whole range, have had voltage/clock speed issues for awhile now that AMD haven't been able to fix with drivers. quote from someone on another forum pretty much sums it up: "The main issue is how the card controls temps, voltages and clock speeds dynamically. Once the card gets hot, it starts throttling itself by dropping the voltage and clock speeds. Sometimes it does this too abruptly and then crashes or freezes the system."
I ran MBT in a window with MSI afterburner running along side just to check how the GPU reacts to different scenarios; it looks as though on idle my GPU clock will sit at around 300mhz, and Mem clock at 150mhz; when playing a video in MBT the GPU clock will increase to 450mhz and mem to 1400mhz; but when BROWSING MBT or during image transitions on the home screen etc, the GPU clock will kick up to 1150mhz, and my GPU temperature will actually raise 10 degrees just with MBT open and idle. I think the crashing may be caused by the R9 series dynamic adjustments issue - and MBT causing it to be so up and down every few seconds.
A workaround was to prevent dynamic voltages being sent to the card by installing Afterburner (with no CCC installed), and using this to control clock speeds, disable ultra low power state options, and enable 'force a constant voltage'. it's been pretty dicey so far as i've had to sit here and work out gradually increment by increment what my 'sweet spot' is before it'll crash out on me; but with this last adjustment i've had MBT running a couple of hours, watched a movie, back to the UI, and had it open for a few more hours; with no issues so far.
what a pain.